As for earthquake-proofing systems for structures including buildings, laminated rubber bearings have come into wide use, and they are classified into three types.
A first type, as shown in FIGS. 29(a) and (b), is a laminated rubber bearing X, wherein rubber plates 1 which are low in compression permanent strain, such as natural rubber, and steel plates 2 are alternately laminated and fixed together. Since this type has a high ratio of vertical compression rigidity to horizontal shear rigidity, it reduces transmission of earthquake energy to a structure while stably supporting the structure, which is a heavy object, against earthquakes.
A second type is a lead-laminated rubber bearing Y (Japanese Patent Publication No. 17984/1986) which is a modification of the laminated construction used for the first type of laminated rubber bearing, incorporating a lead plug 3, as shown in FIGS. 30(a) and (b), vertically inserted therein. Thanks to hysteresis damping provided by plastic strain of the lead inserted in the interior as indicated by a load-displacement curve shown in FIG. 31, this type reduces the amplitude of vibration of a structure produced by an earthquake and quickly damps the vibrations.
A third type is a highly damping laminated rubber bearing Z, which is a modification of the laminated rubber bearing X shown in FIGS. 29(a) and (b), wherein the laminate itself is given a damping property by using highly damping rubber for rubber plates 1.
However, the aforesaid laminated rubber bearings X, Y and Z have the following respective problems.
The first type of laminated rubber bearing X has a vibration damping property which is so low that the direct use of the same will result in an increased amplitude of vibration of a structure during an earthquake; thus, the bearing lacks safety. Therefore, usually, in use it is combined with a separate damper disposed in parallel therewith. In this case, the point of action of restoring force does not coincide with the point of action of damping force, so that there is the danger of giving unnecessary torsional vibrations to the structure.
In the second type of lead-laminated rubber bearing Y, the lead plug 3 develops a high initial shear rigidity for slight vibration, as shown by a characteristic S in FIG. 31; thus, the bearing has poor vibration proofing performance such that it transmits traffic vibrations produced by passage of vehicles. Therefore, it can hardly be applied to a building or floor for installing machines where vibrations are objectionable. Another problem is that the restoration to the original point subsequent to substantial deformation is retarded by the plasticity of the lead.
In the third type of highly damping laminated rubber bearing Z, the amount of creep of the highly damping rubber is high and its restoring force associated with horizontal displacement is low; thus, there is a problem that the reliability for prolonged use is low. Further, the amount of creep differs from one highly damping laminated rubber bearing to another, so that as a result of the earthquake-proofing action, the building shows non-uniform subsidence, causing unnecessary stresses to be produced in the structure.